Many lines will be closed and trains only running between 7am and 7pm with short notice cancellations are also possible.
Strike action will affect travel to the Southampton Pride event and Southampton FC’s fixture against Queen’s Park Rangers at St Mary’s Stadium at 3pm.
Customers should note the last train times across the network, which will be earlier than usual.
The last train from Southampton Central towards London Waterloo will depart at approximately 5.45pm.
Journey planners are now up to date for Saturday, August 26 and the very latest information will be available at southwesternrailway.com/strike.
Customers should check their entire journey before travelling as other operators will be affected.
The ASLEF union has announced strike action on Friday 1 September and a ban on overtime working on top of the RMT union’s strike action on Saturday 2 September. Customers are once again advised to check the SWR website for the latest information.
Stuart Meek, Chief Operating Officer at South Western Railway, commented: “We’re sorry that customers will once again face disruption due to strike action by the RMT union.
“Once again, I must ask our customers to only travel if absolutely necessary on Saturday 26 August.
“If you do have to travel, please check your entire journey as these strikes also affect other train operators.
“A large number of events take place on Saturday 26 August and while some trains will be running, we expect them to be very busy.
“If you’re travelling to one of those events, please check before you travel and consider alternative transport if possible.
“We’re thankful for our customers’ patience during these latest RMT strikes.”